1. Field of the Invention
The present invention relates to a security thread for security paper such as banknotes and to film which can be cut into security threads. The present invention also relates to a method of manufacture of a security thread suitable for use in security paper such as banknotes and other security articles.
2. The Prior Art
Our GB-A-1127043 describes security devices, such as security threads, comprising magnetic material. Such devices allow banknotes and other documents to be authenticated on high speed used note sorting machines and other devices by verification of the presence of the magnetic component.
Also, our GB-A-1585533 describes other security devices which combine a machine verifiable layer of magnetic material with another layer of a non-magnetic metal or luminescent substance, such other layer being in itself machine-detectable. Banknotes containing security devices conforming to the above two patents have been in widespread use for many years; accordingly, there are many banknote sorting machines around the world already fitted with detectors for magnetic security threads as mentioned above.
Furthermore, our EP-B-0319157 describes security paper containing a security thread which is predominantly metallised but has clear regions, at least some of which are wholly surrounded by metal, forming a repeating pattern, e.g., in the form of the characters of an alphabet. This is a strong public security feature and has been adopted by the banknote issuing authorities in many countries; this feature has become known in the art as the Cleartext feature. Furthermore, U.S. Pat. No. 4,652,015 describes paper with a security device with isolated characters of metal; security paper of this type has been used for a recent issue of the United States currency.
There is increasing interest from banknote issuing authorities in combining into the same security device the benefits of a strong public security feature with the covert properties of a machine-readable feature. In particular, there is a need to combine the very strong public security of the Cleartext feature described in EP-B-0319157 with the magnetic properties of the devices described in our two above-mentioned GB-A-1127043 and GB-A-1585533 such that the resultant security device is directly compatible with the widely established magnetic thread detectors already in use around the world.
The security device of PCT application WO92/11142 is an attempt to provide this combination. A security device conforming to this specification has been used commercially. A central region of the security device has a metallic appearance with clear regions forming characters; on either side of this central strip in the width direction, there are layers of magnetic material with obscuring coatings to provide the necessary magnetic component. This is, however, a generally unsatisfactory means of achieving the combination of the appearance of Cleartext with the required magnetic properties. The resultant thread is wide (2.0 mm or more) which presents processing problems to but the papermaker and banknote printer. The magnetic properties are satisfactory, but the requirement to place the magnetic layers on either side of the central region means that the latter must be relatively narrow with respect to the overall thread width and results in characters which are small --typically 0.7 mm high--and therefore not easily legible. Additionally, the structures of the devices described in WO 92/11142 are very complex and present substantial lateral registration problems in depositing the various layers; misregister of even 0.1 mm or can allow the presence of the dark magnetic oxide to be apparent to the naked eye, thus revealing its presence and seriously detracting from the aesthetic appearance of the security thread.
A more satisfactory solution from the points of view of processability, ease of character recognition and aesthetics would be to manufacture a device of the kind described in EP-B-0319157 from a metal which is itself magnetic, such that the size of characters and ratio of character height:thread width of the Cleartext product is maintained, while providing direct compatibility with existing magnetic thread detectors. One means of achieving this is disclosed in Research Disclosure issue 323, page 178, of March 1991. In this disclosure, a magnetic metal is deposited onto a flexible substrate for example by vacuum sputtering; the non-metallised regions are created by selective printing of a resist and subsequent chemical etching. The disclosed magnetic metals may be nickel, cobalt, iron or alloys thereof with a preferred combination of cobalt:nickel in the ratio 85:15. The disadvantage of this method is that vacuum deposition of cobalt:nickel to the necessary thickness is a relatively slow process and somewhat wasteful of cobalt, which is an expensive material. Furthermore, subsequent to this vacuum deposition process, further significant processing is required to etch the characters. The resultant product is therefore relatively expensive.
It is known that films of cobalt:nickel:phosphorous can be prepared by electrodeposition and of cobalt:phosphorous by electrolytic and chemical reduction (Journal of Applied Physics, Vol. 36, No. 3 March 1965, page 948). This paper describes the preparation of films of cobalt:nickel:phosphorous by chemical reduction (electroless plating) using a tin chloride:palladium chloride catalyst. The paper also shows that the magnetic coercivity is strongly dependent upon the nickel content of the alloy. Another paper on the electroless deposition of cobalt phosphorous films has shown that the coercivity is dependent upon the phosphorous content (Journal of the Electro Chemical Society, April 1966, page 360). Again, activation of the substrate involves a catalyst based on tin chloride:palladium chloride. In both of the above papers, a continuous magnetic metallic film is generated (continuous on a macro scale).
Electroless deposition of cobalt on polyethyleneterephthlate (PET) sometimes called Mylar (a trade mark), a non-conductive substrate, is described in the Journal of the Electrochemical Society, June 1962, page 485. In the experimental procedure described, Mylar was immersed in an adhesive and then successively in stannous chloride and palladium chloride solutions prior to deposition of the cobalt layer. The resulting film was suitable for use in high-density data storage applications.
U.S. Pat. No. 5,227,223 discloses a process for electrolessly depositing metal on to a pattern of catalytic material printed on to a moving web of polymeric film so as to form electronic circuits on the film or electrical components or micro-engineering components. The process provides metal images having fine dimensions, e.g., as low as 25 .mu.m or less. U.S. Pat. No. 5,227,223 makes mention of several prior specifications which use electroless deposition to produce printed circuits. These prior specifications all discuss the deposition of some metals which are non-magnetic as would be expected in the manufacture of electrical currents where magnetised components would be disadvantageous. The preferred embodiment of the process of U.S. Pat. No. 5,227,223 uses a nickel bath and deposits nickel onto a substrate by electroless deposition; nickel deposited in such a manner is non-magnetic.